Paper mills have for many years made extensive use, for the cleaning of paper making stock, of screening apparatus embodying a cylindrical perforated screening member defining supply and accepts chambers on the opposite sides thereof in a closed housing, and including a rotor member which operates in one of the chambers to keep the screening perforations open and free from solid material tending to cling to the screening surface. Commonly, the stock or furnish is delivered to the supply chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collected and discharged from the opposite end of the supply chamber.
The assignee of this invention has manufactured and sold many such screens in accordance with a series of U.S. Pat. Nos., commencing with Staege 2,347,716, and followed by Martindale 2,835,173, Seifert 3,849,302 and 4,105,543, and Chupka-Seifert 4,155,841. Starting with the construction shown in the Martindale patent, all such screens manufactured and sold by applicant's assignee have been characterized by a rotor comprising bars or vanes of airfoil section moving in closely spaced but non-contacting relation with the surface of the screening cylinder for the purpose of creating alternating positive and negative pressure waves effective to prevent plugging of the perforations in the screening cylinder.
The art has experimented widely with detailed variations in screens of the above type, including variations in the vane shape and other forms of rotor, and also in the size, configuration, and spacing of the perforations in the screening cylinder. Thus since the advent of the Staege patent in the mid-1940's, many screening cylinders have been fabricated with multiple uniformly cylindrical drilled perforations, which commonly range in diameter from approximately 0.050 inch to 0.125 inch.
In more recent years, the trade has been offered pressure screens generally of the above type wherein the perforations in the screening cylinder are elongated slots rather than round holes, with the slots running either cicumferentially or axially of the cylinder. Typical such constructions are shown in Lamort U.S. Pat. No. 3,617,008, Holz U.S. Pat. No. 3,581,983, and the above noted Seifert '302 and Chupka-Seifert patents.
Both of the Lamort and Chupka-Seifert patents also show, in addition to slotted cylinders, a plurality of shoulders or small bars running generally axially of the screen cylinder in circumferentially spaced relation around the inlet side of the cylinder, and both also show the rotor vanes on the inlet side of the cylinder. This arrangement is described by Lamort as preventing clogging of the screening slots by fiber, albeit in an undescribed manner. In the Chupka-Seifert patent, the purpose of the bars is described as to generate a field of high intensity, fine scale turbulence in the stock adjacent the inlet side of the screen cylinder and thereby to effect screening of paper fiber stock with minimum fractionation thereof on the basis of fiber length.
The disclosure of the Chupka-Seifert patent is limited to screening cylinders provided with circumferentially extending slots of a width range of only 0.001-0.008 inch. The later Chupka-Seifert application Ser. No. 145,654 filed May 2, 1980, of which the present case is a continuation-in-part, discloses the use of a similar multi-bar arrangement in a screening cylinder having circumferentially extending slots of a substantially greater range of widths, i.e. as wide as 0.030 inch although the preferred width range is stated to be 0.014-0.022 inch, with resulting increase in the capacity of the screen in terms of both tonnage per unit of time and the power requirements per unit of accepted fiber.